In biochemistry a porphyrinogen is a member of a class of naturally occurring compounds with a tetrapyrrole core, a macrocycle of four pyrrole rings connected by four methylene bridges. They can be viewed as derived from the parent compound hexahydroporphine by the substitution of various functional groups for hydrogen atoms in the outermost (20-carbon) ring. Porphyrinogens are intermediates in the biosynthesis of porphyrins, cofactors with a porphine core which are found in many enzymes and proteins including myoglobin, hemoglobin, cytochromes, and chlorophylls.